In general, blow-molded articles are required to have excellent processability, mechanical properties, and environmental stress-cracking resistance. Therefore, there has been a demand for a technology for preparing a polyolefin which satisfies a high molecular weight, a broader multimodal molecular weight distribution, and a uniform comonomer distribution to be preferably used for blow-molding or the like.
Meanwhile, since metallocene catalysts including a Group 4 transition metal are used to easily control the molecular weight and molecular weight distribution of polyolefins, and control a comonomer distribution of polymers, compared to the known Ziegler-Natta catalysts, they have been used in the preparation of polyolefins having both improved mechanical properties and processability. However, there is a drawback that polyolefins prepared using the metallocene catalysts show poor processability because of a narrow molecular weight distribution.
In general, polymers having broad molecular weight distribution exhibit great reduction in viscosity with an increasing shear rate, and thus exhibit excellent processability in the processing area. Polyolefins prepared by metallocene catalysts show high viscosity at a high shear rate due to a relatively narrow molecular weight distribution, etc., and thus there are drawbacks that a high load or pressure is applied during extrusion to reduce extrusion productivity, bubble stability is greatly reduced upon a blow-molding process, and the blow-molded articles have non-uniform surfaces to reduce transparency.
Accordingly, a cascade reactor having a plurality of reactors has been used in order to obtain a polyolefin having a wide multimodal molecular weight distribution using the metallocene catalyst, and attempts have been made to obtain a polyolefin satisfying a wider multimodal molecular weight distribution and a higher molecular weight at the same time through each polymerization step in a plurality of reactors.
However, proper polymerization does not occur in a latter reactor depending on a polymerization time in a former reactor due to high reactivity of the metallocene catalyst. As a result, it is difficult to prepare a polyolefin satisfying a high molecular weight and a wider multimodal molecular weight distribution at the same time. Accordingly, there is a continuous demand for a technology capable of more effectively preparing a polyolefin which has a high molecular weight and a wider multimodal molecular weight distribution, thereby satisfying mechanical properties and processability at the same time and being preferably used for blow-molding or the like.